Saving
a gallon by installing a low-water toilet is different. If it uses a
gallon less per flush, that means one less gallon withdrawn from the
system in the first place, as well as one less gallon returned to the
river. The effect on our net consumptive use is effectively zero. There
are good reasons to do the low-flow toilet anyway — energy consumption
in pumping and treating the water, line losses, cost. Best just to
leave that gallon where it is, rather than take it out and put it back
again. But things like low-flow toilets and other indoor water
efficiencies don't save as much water, in terms of our overall water
budget, as people often think.

Whereas if we quit using water to flush our waste in the first place it would have a huge impact. Composting toilets are explained on this site called Pooh Solutions.

But people get all squeamish when you talk about this. Slate had a story about the relative health benefits of squatting versus sitting and you could practically hear people screwing up their faces as they read.